In one prior art example, a typical rocker arm structure comprises: an arm for transmitting an acting force to one end thereof (said end hereinafter referred to as action transmitting end) adapted to transmit the action to a valve in order to open/close the valve; a support shaft projecting laterally from the arm at a position of the arm offset from said action transmitting end towards the other end of the arm. An annular roller rotatably mounted on a periphery of the support shaft via a rollable bearing equipped with needle bearings, the annular roller adapted to roll when subjected to a cam force that acts on an outer periphery of the roller and adapted to transmit the cam force to the action transmitting end as an acting force that acts on the valve.
With this rocker arm installed in a valve mechanism, the rocker arm structure can appropriately transmit the cam force to the valve mechanism with a reduced frictional loss of power owing to the needle bearings that reduce sliding friction between the cam and the rocker arm (or roller), thereby facilitating improvement in fuel efficiency and engine output.
However, since the above mentioned rocker arm structure involves needle bearings disposed between the support shaft and the roller, the shaft is subjected to the cam force via the needle bearings, which implies that the support shaft is subjected to a localized load. In order to avoid an excessive localized load from acting on the support shaft, the entire length of the needle bearings and the axial length of the roller are made larger than predetermined lengths so as to ensure desired contact areas for transmission of the cam force. The roller, therefore, cannot be shortened less than the predetermined axial lengths so long as needle bearings are employed.